Earthquake is one of the natural disasters bringing major losses of life and property to the human beings, and overlarge residual deformation of a structure is more possible to collapse in an aftershock due to P-Δ effect. Long-span bridges, super high-rise buildings, hospitals, explosive and poisonous buildings, and other important buildings are required to have a certain post-earthquake function except for the security guarantee in earthquake, and can be quickly recovered. After an ordinary reinforced concrete structure is yielded, due to the elastoplasticity of the steel bar, deformation is sharply increased while the improvement on the load carrying capacity is limited; the post-yield stiffness thereof approaches to zero or even negative, thus causing two defects: firstly, under a stable load larger than the yielding load, column damage will be uncontrollable, and the damage is mainly concentrated on a plastic hinge zone near the foot of the column, the post-earthquake residual deformation will be too large, the repair after earthquake is difficult, and it is easier to collapse in an aftershock; and secondly, under the excitation of different earthquake inputs, the post-earthquake residual displacement is separated due to the uncertainty of plastic development, which brings difficulty to quantitative evaluation to the structure damage and risk prevention.
Predication on the residual deformation of the structure under the effect of the earthquake starts to be concerned and considered in the performances based design (PBD), and novel structure systems and new materials are also introduced into earthquake-resistant structure design. Good repairability requires a newly-built structure has the following post-earthquake features: firstly, major components of the structure such as the column are still kept in a good status to satisfy the design idea of strong column and weak beam; and the loss of life and property is little; and secondly, the post-earthquake residual deformation is small, and the repair is quick. A quick post-earthquake repair is especially required for arterial traffics, core buildings and other buildings with high important grade. Studies find that an elastic-plastic system with a hardening feature, i.e., dynamic hardening stiffness in the hysteretic behavior after being yielded has a great impact on the residual displacement of the structure, and using a material with the hardening feature or designing a cross section with stable post-yield stiffness can effectively increase the anti-earthquake response stability and reduce the post-earthquake residual displacement. There are several ways to increase the post-yield stiffness of the structure from the aspect of the cross section of the component: firstly, a material with relatively high stress-strain hardening feature is used; and secondly, the cross section is configured with a reinforcing material having different material properties (such as: mixing of FRP bars and ordinary steel bars, and hybrid FRP bars).
Zhishen Wu and Gang Wu et al studied a hybrid FRP reinforced concrete structure early, and proposed the possibility and necessity of realizing the design of the post-yield stiffness from material to structure, and developed a steel-fiber reinforced polymer composite bar (SFCB) and SFCB reinforced concrete structure thereof. The inner core of an SFCB is steel bar or steel wires, and the outer layer is longitudinally composited with FRP, so that the advantages of the two can be complemented. Since the FRP has the features of high strength, low elastic modulus, poor ductility, good durability and light weight, while the steel material has the features of low strength, high elastic modulus, good ductility, poor durability and heavy weight, the two are strongly complemented, and the SFCB obtained has stable and controllable post-yield stiffness after being yielded. Compared with a steel bar, the density of the SFCB is greatly reduced; compared with the FRP, the stiffness of the SFCB is greatly increased, and the cost is much lower; moreover, the fiber and resin outside the SFCB can also play a role of preventing the steel bar of the inner core from corrosion prevention.
The concrete column longitudinally reinforced by SFCBs has the features as follows. Firstly, under the service loads or the effect of small/moderate earthquakes, the SFCBs does not change the natural vibration period of the structure, has the same strength as that of a common reinforced concrete structure, and the high elasticity modulus of the steel bar of the inner core of the SFCB is fully used. Secondly, the externally covered FRP with linear elasticity makes the structure has stable post-yield stiffness on the aspect of the cross section, i.e., after the inner core steel bar of the SFCB is yielded, the high strength of the externally covered FRP enables the bearing capability of the concrete column to be continuously increased to have the post-yield stiffness. This feature can prevent overlarge plastic deformation in the small scope of the foot of the column, realize even distribution of the curvature in a longer area, and reduce the required curvature of the cross section, so that the plastic strain of the inner core steel bar of SFCB is correspondingly reduced. Thirdly, using the SFCB to replace the common steel bar enables the structure to have the feature of certain durability, which has obvious advantages under high-corrosion and other severe environments than ordinary RC structure. In addition, the bonding performance between the SFCB and the concrete can be controlled, and the technology is simple, which can be used to increase the seismic performance of the structure.
The existing SFCB reinforced concrete structure has the following problems.
The ductility is relatively poor; because the limiting strain of the FRP is generally low, it is difficult to satisfy the high ductility requirement of the structure reinforced.
The ordinary steel bar is still used for confinement (stirrups) in the existing SFCB reinforced concrete structure, and the durability cannot be satisfied yet. However, the target of high durability can be realized by using an FRP hoop and a longitudinal SFCB reinforced concrete column, but due to the linear elasticity feature of the FRP, if the FRP hoop reaches an ultimate strength, a brittle shear failure will occur.
It is still difficult to repair the SFCB reinforced concrete column after earthquake, and under great earthquake, if the rupture of FRP occurs to the concrete column structure with relatively high post-yield stiffness, it is easier to result in structure collapse.